Whoa Nelly. Hang on a sec. I have a fundamental physics question. In
my previous post I was using the inverse square law as a measure of
power lost in transmission, but is it more correct to say that it is
power lost per given area.
be captured by having a rectenna that, in the case of the 180 degree
separation, is the inverse of 9%, namely 11.1 times larger than the
transmitter. And in the case of the 120 degree separation, the
inverse of 24%, namely 4.166 times larger than the area of the
transmitter?
If the above is correct, then my doubts about the feasability of
space power sharing may have to be reconsidered. It is my
understanding that rectennas are constructed of less expensive
components than the SPS. Maybe it could work?
> Whoa Nelly. Hang on a sec. I have a fundamental physics question. In
> my previous post I was using the inverse square law as a measure of
> power lost in transmission, but is it more correct to say that it is
> power lost per given area.
>
The equation of beam spreading is a function of the transmitting antenna
aperture versus the frequency.
The bigger the aperture, the tighter the beam.
Beam spreading does not cause significant power loss. The size of the
antenna is set to precisely match the dimensions of the beam as it
intersects
the Earth's surface.
A little bit of energy is lost due to sidelobes which are caused by
diffraction. It is typically not worth it to make the receiving antenna
large enough to catch all the sidelobes.
>
> The inference from the last question, is that all of the power can
> be captured by having a rectenna that, in the case of the 180 degree
> separation, is the inverse of 9%, namely 11.1 times larger than the
> transmitter. And in the case of the 120 degree separation, the
> inverse of 24%, namely 4.166 times larger than the area of the
> transmitter?
>
I do not understand your numbers, but they do not appear correct.
>
> If the above is correct, then my doubts about the feasability of
> space power sharing may have to be reconsidered. It is my
You need to read up on antenna design, and terms like sidelobes and
diffraction. Inverse square law does not really apply.
>
> understanding that rectennas are constructed of less expensive
> components than the SPS. Maybe it could work?
>
Rectennas are certainly cheaper than transmitting antennas and PV
arrays.
Victoria,
of the original works on beaming power from space. It contains the
basic equations and parameters for the SPS beam spreading and antenna
design.
http://engineer.tamu.edu/tees/csp/wireless/70dec5.htm
In particular, look at Section II-C of this paper.
II. UNIQUE PROPERTIES AND BASIC PRINCIPLES OF BEAMED MICROWAVE
POWER TRANSMISSION
C. Free Space Transmission
Regards,
Charles R.
>
> The inverse quare law does not apply to a focussed beam.
>
> The equation of beam spreading is a function of the transmitting
antenna
> aperture versus the frequency.
>
> The bigger the aperture, the tighter the beam.
knowledge is a dangerous thing :)
Apart from the structural mass involved in increasing the aperature
size, what additional high-tech, low mass components would be needed
to increase aperature size?
>
> Beam spreading does not cause significant power loss. The size
of the
> antenna is set to precisely match the dimensions of the beam as it
> intersects
> the Earth's surface.
>
> A little bit of energy is lost due to sidelobes which are caused by
> diffraction. It is typically not worth it to make the receiving
antenna
> large enough to catch all the sidelobes.
>
> > The inference from the last question, is that all of the power
can
> > be captured by having a rectenna that, in the case of the 180
degree
> > separation, is the inverse of 9%, namely 11.1 times larger than
the
> > transmitter. And in the case of the 120 degree separation, the
> > inverse of 24%, namely 4.166 times larger than the area of the
> > transmitter?
> >
> I do not understand your numbers, but they do not appear correct.
In light of your clarifiaction, the numbers are meaningless. Just an
exercise in flawed logic.
> >
> Apart from the structural mass involved in increasing the aperature
> size, what additional high-tech, low mass components would be needed
> to increase aperature size?
>
Essentially:
1) A parabolic reflector with a single huge transmitting element. The
technical challenge is to keep the accuracy of the parabolic surface to
within about a 1/10th wavelength over the entire surface
2) A flat phased array, with zillions of equally spaced transmitter elements,
usually klystron tubes. Again, the main challenge is to keep the surface
perfectly flat within 1/10th of a wavelength over the entire surface.
As the aperture increases it becomes more difficult to maintain the required
surface accuracy.
So lots of little SPSes would be easier to build than one humungous one.
>
> Victoria,
>
> Take a look at this link to the classic "Bill Brown" paper, which
is one
> of the original works on beaming power from space. It contains
the
> basic equations and parameters for the SPS beam spreading and
antenna
> design.
>
> http://engineer.tamu.edu/tees/csp/wireless/70dec5.htm
>
> In particular, look at Section II-C of this paper.
>
> II. UNIQUE PROPERTIES AND BASIC PRINCIPLES OF BEAMED MICROWAVE
> POWER TRANSMISSION
>
> C. Free Space Transmission
>
> Regards,
>
> Charles R.
folder. Just the kind of material I'm looking for.
I have a question about Section II-D, "Power Handling Capability of
Devices in Space as a Function of Efficiency and Operating
Temperature"
Having looked at the performance issues of solar cell versus solar
thermal power generation, I've developed a perference for the solar
thermal option because of greater efficiency to found with sterling
engines, well established engineering principles, and the lessening
of Van Allen Belt radiation damage to the SPS "solar interface"
material.
Wouldn't it make sense, if indeed a SPS is of a solar thermal
design, to incorporate the waste heat generated from the dc-to-
microwave energy conversion process back into the solar thermal
cycle rather than radiate it into space.
In your opinion, if you accept the premise of the solar thermal
option, wouldn't the radiatiant heat feeback loop seriously diminish
the radiator size on an SPS?
>
> Wouldn't it make sense, if indeed a SPS is of a solar thermal
> design, to incorporate the waste heat generated from the dc-to-
> microwave energy conversion process back into the solar thermal
> cycle rather than radiate it into space.
>
That waste heat is low grade, low temperature. Heat engine efficiency
increases as you increase the temperature difference.
You would get more energy by putting the engine in sunlight than by trying to
use the same engine to capture PV waste heat.
>
> In your opinion, if you accept the premise of the solar thermal
> option, wouldn't the radiatiant heat feeback loop seriously diminish
> the radiator size on an SPS?
>
Not by much.
>
> > > >
> > >
> > Apart from the structural mass involved in increasing the
aperature
> > size, what additional high-tech, low mass components would be
needed
> > to increase aperature size?
> >
> There are a couple of different concepts for antenna design.
>
> Essentially:
>
> 1) A parabolic reflector with a single huge transmitting
element. The
> technical challenge is to keep the accuracy of the parabolic
surface to
> within about a 1/10th wavelength over the entire surface
structure of the SPS would be constructed in orbit of lunar/NEO
material, but that the high tech components would have to come from
earth.
Do you foresee any possible way that the antenna you described in
option #1 could be constructed on earth and launched to the SPS? The
inference I make from reading your description is that the orbital
assembly of such a large parabolic surface from discrete components,
would introduce intolerable errors.
Perhaps the transmitting element is launchable?
Could vacuum vapor disporition in orbit provide the accuracy to the
parabolic form? Perhaps an inflatable mold from which the parabolic
shape is lifted?
1/10 the wavelength is pretty steep isn't it? How close are the
tolerances on telescope mirrors?
>
> 2) A flat phased array, with zillions of equally spaced
transmitter elements,
> usually klystron tubes. Again, the main challenge is to keep the
surface
> perfectly flat within 1/10th of a wavelength over the entire
surface.
Could these be assembled as components on earth and the assembled as
a whole in orbit. Perhaps jigs could be used to achieve the
necessary tolerances? Would the vibration of launch ruin the
tolerances of the component assemblies?
>
> As the aperture increases it becomes more difficult to maintain
the required
> surface accuracy.
Understood.
>
> So lots of little SPSes would be easier to build than one
humungous one.
Does the above statement assume one transmitting attenna per SPS?
Could not a SPS incorporate multiple antenna modules, so that the
one SPS can beam to multiple rectennas and perhaps achieve load
balancing on the SPS itself between the different trans. antennas?
In this way, if the SPS is of solar thermal design, could engines be
run in cycles, so that as more power is needed more engines are
brought on line. Other times, engines could be shut down for
servicing or swapping out, without shuttting down all of the power
transmission to the various rectennas?
As you pointed out in a previous post, the beam efficiency varies
depending on the angle of capture at the rectenna. With the limited
amount of "slots" in GEO wouldn't it make sense to make a super-
sized SPS, attach multiple antennas, and beam to multiple rectenna
farms over a land area stretching 10-20 degrees longitude?
victoriatangoman wrote: Wouldn't it make sense, if indeed a SPS is of a solar thermal design, to incorporate the waste heat generated from the dc-to- microwave energy conversion process back into the solar thermal cycle rather than radiate it into space. No, not really. More than that- really not. The waste heat is an inevitable byproduct of making electricity. We can produce energy only because we are able to radiate away the waste- otherwise our satellite would fry for one thing. For a given temperature, and a given waste heat temperature there's an efficiency you can calculate. The rest is waste heat, and must be dissipated away. It's typically 45-70% waste heat with power stations on earth.
That waste heat is low grade, low temperature. Heat engine efficiency increases as you increase the temperature difference. In your opinion, if you accept the premise of the solar thermal option, wouldn't the radiatiant heat feeback loop seriously diminish the radiator size on an SPS? Well, if kept up it would vapourise everything, including the radiator, if that's what you meant!
I've got a question for all you brainiacs - how would you calculate
the wind velocity of a window panel blowing out in a Habitat?
factors.
I got to thinking about the process of replacing the window panel
and thought that the crew will have to be working in a turbulent
vortex as they manoeuver the surface area of the replacement window
into place.
Follow-up question - what would be your preferred process for window
replacement? Secure from inside the Habitat or from outside?
TangoMan
> I got to thinking about the process of replacing the window panel
> and thought that the crew will have to be working in a turbulent
> vortex as they manoeuver the surface area of the replacement window
> into place.
Or perhaps they would seal off an entire section? I can't see
anybody working in those windy conditions. Especially when it isn't
necessary.
> Follow-up question - what would be your preferred process for
> window replacement? Secure from inside the Habitat or
> from outside?
Depends on the method of sealing off. If sealed using a temporary
"half-bubble" for example, I would want the windows secured from the
outside. (So my repair crew isn't trapped in the bubble.) OTOH, if
the windows are secured from the inside, perhaps a simple replacement
window could be devised that drops into place with the pressure of
the habitat.
Notice I avoided the brainiac stuff. :)
Chris.
=====
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Depends on the method of sealing off. If sealed using a temporary
"half-bubble" for example, I would want the windows secured from the
outside. (So my repair crew isn't trapped in the bubble.) How about a "half-bubble" with an airlock on the side? Of course we'd be discussing repairmen in pressure suits. One comment should madeon this topic. I don't know the math to calculate wind speeds, but I have heard somebodyremark that while the airflow may well be supersonic right at the opening, airspeeds just a few feet away would only bea fewmiles an hour. In which case, Hollywood has been giving us a fairly distorted picture of reality. (But then don't they always?) How many times have we seen people in this scenario get picked up by the air flow, fly tens of feet through the air, and get sucked out? (correction: that's blown out) But from what this person was saying, you could come up a lot closer to an opening than what many people would have anticipated if their only notion of this is what they've seen in the movies.
Mike Combs
> How about a "half-bubble" with an airlock on the side? Of course we'd
> be discussing repairmen in pressure suits.
> One comment should made on this topic. I don't know the math to
> calculate wind speeds, but I have heard somebody remark that while the
> airflow may well be supersonic right at the opening, airspeeds just a
> few feet away would only be a few miles an hour.
I shouldn't be surprised if this is true. A vacuum cleaner can pick up a fairly dense and heavy object if the mouthpiece is placed directly over it, but leave dust undisturbed that lies a few millimeters away from the mouthpiece. And when you drain a sink, the soap suds away from the vortex barely move.
The distance between safe and doomed may be quite short. The repair crew should have safety equipment like that of mountaineers. Once the dome is over the hole but before it has been pushed fully into place and the wheels have been retracted, there should be quite a wind between the skirt and the glass surface, unless the dome is large compared to the hole.
If you have a pressurized tube where one end cap is suddenly removed, I should expect the wind speed to be inversely linear with the distance to the hole. With a pie-shaped pressure vessel, I should expect an inverse square relationship, once you are far away from the hole compared to the thickness of the pie. With the pressure vessel extending in three dimensions I should expect an inverse cube. These of course at distances large compared to the size of the hole.
And just as roads are often left in bad repair, full of holes after the winter, because the authorities are short-funded (or think that they are), you might expect a few such domes left over shattered panels until the authorities can afford to have them fixed.
Jon Lennart Beck.